The petroleum industry is generally categorized into three parts: upstream, midstream, and downstream. Upstream processes are generally considered to be the processes that are conducted in the oilfield and are most commonly used to refer to the processes of searching for and recovering crude oil and/or natural gas from the Earth. Crude oil is a fossil fuel containing hydrocarbons and other organic liquids and is most commonly found below the Earth's surface. Typically, as it is difficult to separate during extraction, crude oil is obtained with many other fluids found below the Earth's surface, including water and gases. Initial separations at the oilfield may be used to separate the water, oil, and gases based on density differences, such as via settling tanks and other vessels or process equipment. The extracted crude oil is then processed and refined into useful and more valuable products such as diesel fuel, gasoline, kerosene, etc.
The processing and refining of crude oil is conducted at an oil refinery and is an essential part of the downstream side of the petroleum industry. During the refining process, the hydrocarbons may be processes in mixture with gases, solids, and or other liquids, including water. While in a storage or process vessel, the process fluid(s) may undergo stratification (layering) under the natural force of gravity due to the varying densities of the different substances that make up the process fluid in the vessel. Additionally, in certain processes, an electric field may be applied to the process fluid of the vessel in order to facilitate stratification. The facilitation using an electric field is known as electrophoresis.
Due to the different amounts of crude oil, water, and any other liquids or gases in the process mixtures, at the oilfield, during refining, etc., the layering of the fluids in the vessel is not consistent or constant. Therefore, as the processing may be continuous and ongoing, it is difficult to determine the density profile (i.e., the density of the process fluid contained within the vessel at several elevations along the vessel simultaneously) of the process fluid at any given time.
In many cases, however, it is beneficial to know the density profile of the process fluid at a specific time. Engineers use the density information to determine the percentages of each liquid contained within the process fluid, completeness of separations, etc. In addition, engineers may also use the density profile information to make adjustments to the refining process, control flows to and from vessels, and control of other aspects of the processes. Further, it is important for productivity and quality to know water, oil, and emulsion (mixture) are located within the vessel at any given time. To determine the density profile in a vessel, process vessels may be equipped with a number of sampling ports (taps). Typically, the taps may be disposed at different elevations along the wall of the vessel.
Using the taps, obtaining samples of the process fluid at each tap elevation is convenient. However, taking a sample from each tap and analyzing each sample in the lab is laborious and slow. Therefore, by the time each sample is analyzed in the lab and the density at each elevation is calculated, there may no longer be a need for the density profile information. As such, it may be advantageous and more efficient to employ an array of sensors where each sensor is capable of calculating the density of the process fluid at a particular elevation at any given time.
Like most sensing and measuring tools, each sensor of the sensor array needs to be properly calibrated. Calibration is necessary in order to determine both the accuracy of each sensor's response and, more importantly, to maintain the accuracy of each sensor's measurements. One useful and common method to calibrate the sensor array is done by using a two-point calibration technique. In this technique, the vessel is successively filled with two liquids of different and known densities, for example, water and oil. Afterwards, the response of each sensor in the sensor array is recorded and memorized. Typically, it is beneficial to fill the vessel with liquids whose densities vary greatly between each another. In this respect, the measurement of each liquid will likely result in vastly different sensor responses. Therefore, the change in density that the sensor detects will be much more noticeable in each sensor's response. In operation, the density at specific elevations may then be derived from each sensor's response by interpolating between the densities used in calibration of the sensor array to resulting in a calculated density based on the sensor's response.
Despite the simplicity of the aforementioned two-point calibration or re-calibration technique, it is unpopular in the refining process as it is disruptive to production as production would need to be stopped in order to fill the vessel successively with two liquids of known densities. As mentioned above, it is beneficial and, in most cases, preferred, to have an uninterrupted refinement process. Therefore, process engineers need a method to calibrate or re-calibrate each sensor of the sensor array without halting production.